1. Field of the Invention
The present invention relates to an integrated optical circuit device and a method for producing the same, and more specifically, to a method for coupling a plurality of optical elements. In particular, the present invention relates to an integrated optical circuit device incorporating a semiconductor loser section and an optical waveguide section having different near fields from each other, as well as a method for producing the same.
2. Description of the Related Art
Conventionally, integrated optical circuit device incorporating a semiconductor laser element (also referred to as the xe2x80x9csemiconductor laser sectionxe2x80x9d) and an optical waveguide element (also referred to as the xe2x80x9coptical waveguide sectionxe2x80x9d) having different near fields from each other are produced by a hybrid method which involves separately fabricating a semiconductor laser element and an optical waveguide element having different near fields and then combining the two elements. However, the hybrid method requires very accurate positioning and therefore is not highly industrially applicable, i.e., is not highly adaptable to being efficiently produced.
In order to solve this problem, there has been proposed a method which integrally forms a semiconductor laser element and an optical waveguide element on the same substrate.
For example, Japanese Laid-open Publication No. 63-182882 discloses a structure including an active layer 2 and a cladding layer 3 deposited on an InP substrate 1 as shown in FIG. 14. A portion of the deposited active layer 2 and the cladding layer 3 is etched away, so that a buffer layer 5, an optical waveguide layer 6, and a protective layer 7 are integrally formed in the etched portion. An electrode 8 is formed over the remaining portion of the active layer 2 and the cladding layer 3 so as to function as a semiconductor laser section.
FIG. 15 shows another conventional example of an integrated semiconductor laser and optical waveguide disclosed in IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 2, No. 2, p.88 (February 1990), which includes a semiconductor laser section 13 and an optical waveguide section 11 which are integrally formed with a tapered section 12 interposed between, such that the tapered section 12 provides a gradually varying light distribution.
However, the structure shown in FIG. 14 inevitably results in the formation of a section 9 between the active layer 2 and the optical waveguide layer 6 which lacks the function of light confinement in a vertical direction. Since light cannot be confined along the vertical direction in the section 9, unwanted light radiation occurs not directing towards the optical waveguide layer 6, thereby decreasing the coupling efficiency to the optical waveguide layer 6.
Furthermore, since the active layer 2 and the optical waveguide layer 6 are formed through separate growth processes, it is difficult to align the active layer 2 and the optical waveguide layer 6 along the height direction within the controllability of available crystal growth technology. For example, in the case where the optical waveguide layer 6 is formed by a common metal organic chemical vapor deposition method (hereinafter referred to as the xe2x80x9cMOCVD methodxe2x80x9d), its thickness and position along the height direction may be offset from design values by about 5% to 10%. In general, it is desirable to form an optical waveguide layer so as to have a thickness of several xcexcm to facilitate coupling with external elements (e.g., with an optical fiber), and this results in a buffer layer (underlying the optical waveguide layer) having a thickness of several xcexcm. In such cases, the total positional offset along the height direction may be as great as 0.1 to 0.5 xcexcm, and the thickness of the section 9 emerging between the active layer 2 and the optical waveguide layer 6 may also be on the order of xcexcm, resulting in a coupling loss of about 1 dB (aside from coupling losses due to mode mismatching). The above-described positional offset, or light radiation in a section lacking the light confinement function, becomes especially remarkable in elements having a small light distribution width, e.g., semiconductor lasers.
On the other hand, in the structure shown in FIG. 15, since the semiconductor laser section 13 and the optical waveguide section 11 are coupled via the tapered section 12 providing a gradually varying light distribution, substantially no light radiation occurs between the two sections 13 and 11 so that highly efficient optical coupling may be obtained. However, the formation of the tapered section 12 requires several (e.g., three or more, for the illustrated structure) etching processes and regrowth processes. Such complexity in the production process, as well as degradation in the crystallinity of the tapered section 12 and the optical waveguide section 11 during the regrowth process, detracts from the theoretical effects of this technique so much that no industrial applications of this technique have been reported heretofore.
In addition, the structure in FIG. 15 is also susceptible to some propagation loss due to loss of free carriers within the optical waveguide section 11 because the optical waveguide layer of the optical waveguide section 11 and the active layer of the semiconductor laser section 13 are formed of the same material, indicative of insufficient characteristics as an optical waveguide section 11.
An integrated optical circuit device of the invention includes: a first optical element section including first and second element regions deposited along a thickness direction; a second optical element section formed away from the first optical element section; and a multimode interference region provided between the first and second optical element sections, the multimode interference region including a buried portion formed along a light propagation direction. The first and second optical element sections are optically coupled to each other via the multimode interference region.
The buried portion of the multimode interference region may be arranged so as to have a length such that light outgoing from the first optical element section reaches via translation the second optical element section while retaining a light distribution shape at the time of outgoing from the first optical element section.
A width of the buried portion of the multimode interference region along a direction perpendicular to the light propagation direction may change in one of a gradual manner and a stepwise manner.
A thin interface region may be formed between the first optical element section and the multimode interference region.
According to another aspect of the present invention, an integrated optical circuit device includes: a first optical element section including first and second element regions deposited along a thickness direction and having mutually different light distribution widths; a multimode interference region formed in a position along a propagation direction of light outgoing from the first optical element section, the multimode interference region including a buried portion which has a multimode waveguide structure along each of the thickness direction and an in-plane direction which is perpendicular to the thickness direction; and a second optical element section formed away from the first optical element section with the multimode interference region interposed therebetween. The first and second optical element sections are optically coupled to each other via propagation of the light outgoing from the first optical element section through the multimode interference region to the second optical element section.
In the above-mentioned structure of the integrated optical circuit device, the second optical element section may include a mesa structure which in linearly aligned with the first optical element section and the buried portion of the multimods interference region.
Furthermore, the buried portion of the multimode interference region may include two or more layered subregions.
A method for producing an integrated optical circuit device of the invention includes: a formation step for forming a first optical element section including first and second element regions deposited along a thickness direction concurrently with a second optical element section disposed away from the first optical element section; a first etching step for etching a region between the first optical element section and the second optical element section to form an etched groove; and a burying step for forming a buried portion of a multimode interference region in the etched groove.
In one embodiment, in the first etching step, substantially all of a portion in which the multimode interference region is to be formed is etched away to form the etched groove, and the burying step includes: a step of burying the entire etched groove with material constituting the buried portion of the multimode interference region to form a first buried layer; a second etching step for etching away a predetermined portion of the first buried layer to leave a portion corresponding to the buried portion of the multimode interference region; and a step for forming a thin film or a second buried layer so as to cover sides of the remained buried portion of the multimode interference region.
In one embodiment, the first etching step concurrently forms a mesa structure for light confinement along a transverse direction in the second optical element section.
According to another aspect of the invention, a method for producing an integrated optical circuit device includes: a step of forming a multimode interference region having a buried portion; a step of etching a predetermined region in the vicinity of the multimode interference region so as to form at least two etched-away portions in positions interposing the buried portion of the multimode interference region; and a step of forming a first optical element section including first and second element regions deposited along a thickness direction concurrently with a second optical element section so that the first optical element section is formed in one of the at least two etched-away portions and the second optical element section is formed in the other etched-away portion.
In one embodiment, etching for forming the multimode interference region is performed concurrently with etching for forming a mesa structure for light confinement along a transverse direction in the second optical element section.
In the above-mentioned method of the invention, an etched hole in which an upper electrode of the first optical element section is provided and a ridge of the second optical element section are concurrently formed in the same etching process.
In accordance with the integrated optical circuit device according to the present invention, an optical waveguide layer and an active layer of a semiconductor laser section are sequentially formed on a semiconductor substrate; a portion of the resultant composite is etched away into and below the active layer and the optical waveguide; and a buried portion of a multimode interference region is buried in the etched portion as a layer having such a refractive index as to function as a multimode waveguide in the vertical direction. This buried layer may be interpreted as the multimode interference region in the narrow sense.
The multimode interference region (more strictly, the buried portion thereof) allows the light exiting from the semiconductor laser section to be led into the distant optical waveguide section. Thus, an integrated optical circuit device having a high coupling efficiency can be provided. Since the optical waveguide layers of the optical waveguide section and the semiconductor laser section are isolated along the vertical direction, the optical waveguide section can be produced without being doped. As a result, the light absorption of the optical waveguide section can be minimized.
By optimizing the length of the multimode interference region, it becomes possible to further improve the coupling efficiency between optical elements having different light distribution widths along the direction in which the multiple layers are deposited (hereinafter referred to as the xe2x80x9cthickness directionxe2x80x9d).
Furthermore, when employing a semiconductor laser element (section) and an optical waveguide element (section) as the optical elements, the enhanced effects of the present invention can be obtained since there exists a great difference in light distribution widths between these elements.
Furthermore, by employing an optical buffer layer and a plurality of multimode interference regions, it becomes possible to further enhance the coupling efficiency.
Furthermore, by varying the width of the buried portion of the multimode interference region in a gradual or stepwise manner, it becomes possible to further enhance the optical coupling between optical elements having different light distribution widths along the transverse direction. The effects provided by such an embodiment of the present invention can be further enhanced by ensuring that the rate of change of the width becomes smaller in a region having a smaller width.
Furthermore, by incorporating a multimode interference region functioning as the multimode waveguide along both the thickness direction and the transverse direction, it becomes possible to minimize the influence of manufacturing accuracy of the multimode interference region on the optical coupling efficiency. By adopting a plurality of multimode interference regions along both the thickness direction and the transverse direction using an optical buffer layer, it becomes possible to achieve a high coupling efficiency without being affected by the manufacturing accuracy.
In the case of using a semiconductor laser element (section) as an optical element, an integrated optical circuit device which is capable of operating at a low threshold level can be obtained by providing an insulation layer between the semiconductor laser element (section) and the multimode interference region so as to prevent a current from being injected into the multimode interference region.
Furthermore, all the optical elements according to the present invention can be produced through the same process. Therefore, the relative positioning of the optical elements is facilitated without misalignment along the height direction. Hence, the coupling efficiency is not degraded even in the case of poor processing accuracy.
In the fabrication of the integrated optical circuit device according to the present invention, the formation of a lower electrode of the semiconductor laser section and the formation of a ridge of the optical waveguide section can be performed in the same process, thereby facilitating the fabrication. Alternatively, the fabrication can also be facilitated by performing the burying process of a mesa to be provided for light confinement in the optical element section along the transverse direction and the burying process of the multimode interference region in the same process.
By forming the multimode interference region through the burying process of an etched groove, not through the burying process of an etched hole, a more planar multimode interference region can be obtained resulting in enhanced characteristics.
Preferably, one or more layers for forming the multimode interference region are deposited on a flat substrate, and a portion thereof is etched away while the rest thereof remains to function as the multimode interference region and a plurality of optical elements are then formed. This enables the fabrication of the multimode interference region having substantially flat top and bottom surfaces.
By ensuring that the aluminum mole fraction in a material (e.g., GaAlAs) which is used for a multimode interference region is 0.3 or less, it becomes possible to achieve excellent selective burying result without unwanted deposition of aluminum on a mask or the like. Thus, the resultant multimode interference region performs excellent intended functions.
Thus, the invention described herein makes possible the advantages of (1) providing an integrated optical circuit device in which a plurality of optical elements such as a semiconductor laser and an optical waveguide are produced by the same crystal growth process, with no layer existing between the optical elements that hinders optical coupling therebetween, and (2) providing a method producing such an integrated optical circuit device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.